By CHRISTOPHER GRAY

Published: November 7, 1993

THEY really should leave one patch dirty, because without seeing the facade of 1040 Park Avenue midway through its current cleaning it is hard to imagine how profoundly a layer of soot has obscured the delicate invention of this sophisticated 1924 apartment house.

Apartment development on Park Avenue above 86th Street took off after World War I and every year brought two or three new apartment houses. In 1923 Joseph L. B. Mayer, a developer, began one at the northwest corner of 86th. Although Mayer had used commercial architects on other projects, he retained Delano & Aldrich, better known for upper-class town houses and clubs.

Because you usually can't do a whole lot with what are essentially packing cases for the rich, it is the rare apartment house that has anything approaching inspiration, and 1040 Park is one of these. The architects gave it a chaste limestone base of three floors and an amusing third-story frieze of tortoise-and-hare figures.

At the top is an inventive attic story with an elegant iron balcony of anthemion pattern. This top story also has rounded windows and recalls the corresponding floor on Delano & Aldrich's town houses, like that now occupied by the International Center of Photography at 94th and Fifth Avenue. The building has no cornice, just a frieze of triglyphs and shells, a mild but imaginative mix.

Before the current cleaning, the middle section was a puzzle, apparently bare of any detailing except regular piers of brick laid at a 45-degree angle to form a vertical ridge. The bottom and the top are elegant and clever, but the middle has seemed like the architects had simply gone out to lunch.

But in October the co-op board began a cleaning program. The change has been not just dramatic, but also informative. Years of grime had concealed a sophisticated pattern of brickwork that is only now apparent.

Two soldier courses of brick on each side of the windows shoot straight up. They are very slightly differentiated in color and texture from the rest of the buff facade. Underneath each window is a panel of brick laid end-on. On the cleaned areas there is some sense of composition to the middle section, which now appears as a giant vertical element bounded by the top and bottom sections.

It's a neat trick with some proto-Art Deco feeling, advanced for a period when the standard Renaissance form still held sway. If you have observed 1040 Park Avenue for any length of time, what you have missed will astonish you. The cleaning has also brought out a curious little band of inverted spade-shapes above the animal motif.

There is another thing that is easy to miss -- the glassed-in terrace on the 86th Street side of the building's roof. This was the apartment of Conde Nast, often cited as the first penthouse in New York. It is more likely that it was the 20th or 30th, but it was certainly the most famous in its day, and remains a benchmark for architectural historians.

Nast began his publishing empire in the 1910's and by the mid-1920's was riding high with Vogue, House & Garden, Vanity Fair and other magazines. Nast may or may not have been in on the initial planning of 1040 Park Avenue in 1923, but in 1924 Delano & Aldrich revised the top-floor plans to provide for a huge apartment for Nast with a wide surrounding terrace, an unusual although not unprecedented feature in a luxury apartment house.

Nast must have been one of the first New Yorkers to recognize that an open terrace is of only seasonal use: In 1926 he hired Delano & Aldrich to design a greenhouse-like glass canopy to cover the terrace and had the ballroom extended to the edge of the building.

Three years ago former Parks Commissioner Henry Stern, now Executive Director of the Citizen's Union, founded the American Association for the Advancement and Appreciation of Animals in Art & Architecture -- AAAAAAA for short, or long. In the spirit of full disclosure, I admit I am the archivist of this organization, which has a lifetime membership fee of $1.

The AAAAAAA has adopted for its logo the tortoise-and-hare frieze from 1040 Park Avenue, and on Sunday, Nov. 21, at 11 A.M., Adrian Benepe, Jonathan Kuhn and Mr. Stern ern will lead a free two-hour walking tour of "animal architecture,' starting at 1040 Park Avenue. Nonmembers should reserve a place by telephoning Ian Shapiro of AAAAAAA at 227-0342.

Photos: The view north on Park Avenue near 85th Street in 1930 shows 1040 Park Avenue at 86th Street, center, left, with penthouse. (Municipal Archives); Conde Nast with guests in his penthouse in the 1930's. The penthouse had a wide terrace with glass canopy. (Leslie Bonham-Carter)